I assume people are now selling them directly to companies like PSI or Robot Underground. They can also update your blown subs, so that may be another factor.
I think more companies/overseas builders are building things more proprietary (not all them by any means) but most of the "motors" for sale typically had 12 spoke or ti baskets and were practically universal. While a lot of subs are still very cookie cutter they do try to make getting correct coils/spiders etc harder.